Installation and process for the preparation of hydrogen cyanide

ABSTRACT

Present invention relates to a process and an installation ( 100 ) for the preparation of hydrogen cyanide by the Andrussow process, and more precisely for improving the conditions of mixing the reactant gases before feeding the Andrussow type reactor ( 60 ), in order to improve safety, to avoid any risk of explosion and to produce HCN in safe and efficient manner. The installation is configured in such a manner that oxygen is pre-mixed with air with a ratio comprised between 20.95% and 32.5% by volume, preferably between 25% and 30.5% by volume; methane containing gas and ammonia are simultaneously added in the pre-mixture of oxygen-enriched air in such a manner that the volumic ratio of methane to ammonia is comprised between 1.35 and 1.02 depending on the content of oxygen into air; said obtained reactant gases mixture having a temperature comprised between 80° C. and 120° C., preferably between 95° C. and 115° C. for feeding the Andrussow type reactor ( 60 ).

FIELD OF THE INVENTION

The present invention relates to an improvement of the Andrussow processfor preparing hydrogen cyanide (HCN). It relates more particularly to animproved installation for preparing a mixture of reactant gases flowingthrough the input of an Andrussow type reactor, which contains catalystsgauzes over which the stream of reactant gases mixture flows and reactsto produce HCN.

STATE OF THE ART

Hydrogen cyanide HCN is the starting product for a number of organic andinorganic synthesis, leading for example to the following products:acetone cyanohydrin ACH, cyanuric chloride, adiponitrile, methionine,inorganic compounds such as sodium cyanide and the like, NTA(NiTriloAcetic acid) EDTA (EthyleneDiamineTetraAcetic acid) etc. . . . .When acetone cyanohydrin ACH is synthesized, it is used for example forthe preparation of alkyl methacrylates or methacrylic acids. Suchindustrial processes for obtaining either alkyl methacrylates ormethacrylic acids are made in a continuous manner, and each step of suchprocesses has to be controlled very precisely in order to avoid stopsdue to safety problems relating for example to deflagration risks.

Andrussow's process is a well-known process and widely used for HCNsynthesis by ammoxidation of methane. Hydrogen cyanide is obtained bythe action of ammonia on methane in the presence of air over a catalystconsisting of platinum-rhodium gauzes. The reaction between ammonia andmethane is the following:

CH₄+NH₃→HCN+3H₂  (1)

Such a reaction is endothermic. Therefore, in absence of oxygen, it isnecessary to mix a large quantity of methane compared to ammonia inorder to burn the mixture and bring heat to enable the establishment ofthe reaction inside the reactor. The addition of air enables, thanks tothe combustion of part of the hydrogen produced and excess of methane,to have a generally exothermic system and thus maintain the reactionsynthesis without additional external energy. For that, the threereactant gases (CH₄, NH₃ and air) are mixed in precise volumicproportions. The resulting gas stream is introduced into an Andrussowtype reactor. This reactor comprises catalyst gauzes made ofplatinum-rhodium placed on a support. A heat exchanger is placeddownstream of the reactor for cooling the gas immediately after contactwith the catalyst gauzes. The initiation of the reaction can be forexample carried out with an electrical resistance which lights thegauzes. Once this ignition achieved, overall exothermic reactionsmaintains the gauzes at a temperature of about 750° C. to 1250° C.,preferably 1000° C. to 1200° C.

The main reaction inside the reactor is the following:

CH₄+NH₃+3/2 O₂→HCN+3H₂O  (2)

The mixture of air, methane-containing gas and ammonia is a flammablemixture and the exothermic reaction can lead to an explosion if theproportions of the reactant gases in the mixture are not preciselycontrolled and if the temperature of the mixture and the reaction arenot controlled.

The document US2010/0086468 further discloses that the use of anoxygen-enriched air in the reactant gases mixture increases the reactionyield of production of HCN. However, this document makes safety issuesmore delicate to manage. Indeed, the disclosed ratio of oxygen to air iscomprised between 20 and 100% by volume. Such a range is too large andit is much more awkward to mix pure oxygen or oxygen-enriched air withsuch high ratios of oxygen, with methane and ammonia, and feed anAndrussow type reactor with such a mixture and avoid having anexplosion.

Although the Andrussow process for the preparation of HCN is known sincea while, HCN is not so easy to prepare because the reactant gases ratiosin the mixture have to be adjusted to avoid possible explosion. Thetemperature of the reactant gases mixture has also to be controlled inorder to avoid an explosion of the mixture and a damage of thecatalyst's gauzes of the reactor. Moreover, the reaction inside theAndrussow type reactor being exothermic, there is a risk of theapparition of a backfire from catalyst gauzes into the feeding pipe ofthe reactor.

The document WO97/09273 discloses a hydrogen cyanide process andapparatus therefore. The process uses an oxidant rich stream thatcontains from 30 to 100% by volume of oxygen. This oxygen rich stream ispreheated to a temperature from 200° C. to 300° C. The other streamcalled feed stream is preheated to a temperature range from 300° C. to450° C.

Therefore, there is a need of improving the safety of the continuousindustrial process for production of HCN.

Technical Problem

Present invention aims to avoid at least one of the inconvenient of thestate of the art. More particularly, the invention aims to propose aprocess and an installation for the preparation of hydrogen cyanide bythe Andrussow process, and more precisely for improving the conditionsof mixing the reactant gases before feeding the Andrussow type reactor,in order to improve safety of the continuous industrial process, toavoid emergency shutting down and any explosion and to produce HCN insafe, reliable and efficient manner.

BRIEF DESCRIPTION OF THE INVENTION

Surprisingly, it has been discovered that an installation for thepreparation of hydrogen cyanide HCN, said installation comprising anAndrussow type reactor, in which is introduced a reactant gases mixtureof methane-containing gas, ammonia and oxygen-enriched air, said mixturereacting inside said reactor over a catalyst to produce HCN, saidinstallation comprising, upstream of said Andrussow type reactoraccording to the direction of gas flow, gas feeding pipes for feedingthe installation with each reactant gas, a first unit for pre-mixingoxygen with air before adding methane-containing gas and ammonia intothe obtained pre-mixture of oxygen-enriched air, said installation beingcharacterized in that said first unit comprises at least one oxygenonline controller so as to control the ratio of oxygen into air, saidratio having to be comprised between 20.95% and 32.5% in volume ofoxygen, preferably between 25% and 30.5% in volume of oxygen; saidinstallation comprising a second unit (U2) for adding simultaneouslymethane-containing gas and ammonia into the oxygen-enriched air with avolumic ratio of methane to ammonia, which has to be comprised between1.35 and 1.02 depending on the proportion of oxygen into theoxygen-enriched air; said installation further comprising at least oneheat exchanger for heating at least one of the reactant gases, in orderto have a temperature of said reactant gases mixture feeding saidAndrussow type reactor (60) comprised between 80° C. and 120° C.preferably between 95° C. and 115° C., leads in the production of areactant gases mixture, ready to feed the Andrussow type reactor, whichis completely safe and does not risk to explode.

The applicant has also discovered that a process for preparing hydrogencyanide HCN by reacting, over a catalyst placed inside an Andrussow typereactor, a reactant gases mixture containing a methane-containing gas,ammonia and oxygen-enriched air, the process comprising, before feedingthe Andrussow type reactor with the reactant gases mixture, a pre-mixingstep of oxygen with air followed by a simultaneous addition ofmethane-containing gas and ammonia into the obtained pre-mixture ofoxygen-enriched air, said process being characterized in that oxygen ispre-mixed with air with a ratio comprised between 20.95 and 32.5% byvolume, preferably between 25 and 30.5% by volume; methane containinggas and ammonia are simultaneously added in the pre-mixture ofoxygen-enriched air in such a manner that the volumic ratio of methaneto ammonia is comprised between 1.35 and 1.02 depending on the contentof oxygen into air; said obtained reactant gases mixture having atemperature comprised between 80 and 120° C., preferably between 95 and115° C. for feeding the Andrussow type reactor, leads in the productionof a reactant gases mixture, ready to feed the Andrussow type reactor,which is completely safe and does not risk to explode. Such processenables also to produce a mixture which leads to improve the yield ofthe reaction of production of HCN inside the reactor.

According to another aspect, the invention relates to a process forpreparing a monomer selected from methacrylic acid and/ormethylmethacrylate comprising the steps of:

-   -   preparing hydrogen cyanide HCN by an Andrussow process,    -   preparing acetone cyanohydrin ACH from hydrogen cyanide and        acetone,    -   hydrolyzing acetone cyanohydrin ACH to produce an hydrolysis        mixture comprising α-hydroxyisobutyramide (HIBAM),        α-sulfatoisobutyramide (SIBAM), 2-methacrylamide (MACRYDE) and        methacrylique acid (MAA),    -   thermally converting said hydrolysis mixture into a cracker        mixture to produce a mixture of 2-methacrylamide (MACRYDE) and        methacrylic acid (MAA),    -   reacting the cracker mixture with a material selected from        methanol or water to produce respectively a monomer selected        from methyl methacrylate (MMA) or methacrylic acid (MAA), and    -   purifying said obtained methyl methacrylate (MMA) or methacrylic        acid (MAA),        said process being characterized in that the first step of the        preparation of hydrogen cyanide HCN is processed according to        the process described above, by using installation described        above.

INTRODUCTION OF THE FIGURES

Other features and advantages of the invention will become apparent byreading the following description given by way of illustrative andnon-limiting examples, with reference to the accompanying Figures inwhich:

FIG. 1 represents a ternary diagram of the reactant gas mixturecomposition for a volume ratio of methane to ammonia equal to 1, in amixture according to prior art,

FIG. 2 represents a ternary diagram of the reactant gas mixturecomposition for a volume ratio of methane to ammonia equal to 1.024 in amixture according to the invention for avoiding explosion,

FIG. 3 represents a schematic bloc diagram of an installation forpreparation of the reactant gases mixture before its entering into theAndrussow type reactor,

FIG. 4 represents a simplified schematic bloc diagram of a plant forpreparing a monomer selected from methyl methacylates and/or methacrylicacids.

SPECIFICATION Concerning the Preparation of HCN

FIG. 3 shows a schematic bloc diagram of an installation 100 used toprepare the reactant gas mixture before its entering into the Andrussowtype reactor 60.

This installation 100 comprises four feeding pipes P1 to P4. A firstfeeding pipe P1 is dedicated for the feed of air, a feeding pipe P2 forthe feed of oxygen, a third feeding pipe P3 for the feed ofmethane-containing gas, and the fourth feeding pipe P4 for the feed ofammonia.

Air is directly drawn up from the atmosphere and compressed, by means ofa compressor referenced 11 on FIG. 3, to a pressure comprised between1.4 and 3 bar (absolute pressure).

The installation 100 comprises a first unit, referenced U1 on FIG. 3,for premixing compressed air with oxygen in order to obtain apre-mixture of an oxygen-enriched air. The proportion by volume ofoxygen in relation to the total volume of air has to be preciselycontrolled in order to have a non-flammable pre-mixture. The proportionby volume of oxygen in relation to the total volume of air isadvantageously selected in the range of 20.95% to 32.5%, and preferablybetween 25% to 30.5%.

It has been found that, with such composition, the safety problem ismastered and the flammability area is avoided if the other flow rate andthermodynamic parameters are also well controlled.

For a better comprehension, FIG. 1 shows a ternary diagram of thereactant gas composition in a mixture according to the previously citeddocument US 2010/0086468, and FIG. 2 shows a ternary diagram of thereactant gas composition in a mixture according to the invention. In thefirst diagram of FIG. 1, the two dotted lines represent respectively thelower (LIE) and higher (LSE) flammability limit for a mixture of methaneand ammonia with a ratio equal to 1. The interval between the two greylines corresponds to the operating area of the cited prior art. Such anoperative area is too large because it authorizes to mix pure oxygen, oralmost pure oxygen, with methane containing gas and ammonia. Howeversuch a mixture is not stable because flammable and it risks exploding atany time.

On the contrary, the invention limits the ratios of the reactant gasesin the mixture in precise ratios in order to avoid the explosion area.The two black lines in the second diagram of FIG. 2, correspond to linesalong which air comprises respectively 20.95% by volume of oxygen and30.51% by volume of oxygen. Then between these two lines, an optimalsafe operating area, which appears in dashed line on FIG. 2, is definedby further adjusting the ratios of methane and ammonia. The two dottedlines represent respectively the lower (LIE) and higher (LSE)flammability limit for a mixture of methane and ammonia with a ratio of1.024. The grey line is a stoichiometric burn line. The point referencedLOC corresponds to oxygen concentration limit for the combustion of themixture. Such limits may vary depending on the proportion of oxygen inthe mixture, on the proportion of methane and ammonia, and on thetemperature.

Therefore the selected optimal operating area along the dashed line isan optimal safe area for preparing the reactant gases mixture designedto be fed to the Andrussow type reactor for producing HCN. Such area issure in terms of flammability and explosiveness.

Thus, at the beginning of the preparation of the reactant gases mixture,the first pre-mixture of air and oxygen is made by introducingprogressively oxygen into compressed air in such a manner that thecontent of oxygen is progressively increased until reaching the maximumvalue of 32.5% by volume, and more preferably 30.5% by volume, in orderto have an optimal productivity in safe conditions.

Such proportion of oxygen into air brings another advantage over priorart because less oxygen in air leads to an economic optimum.

In the unit U1 of the installation 100 (FIG. 3), the oxygen flow isregulated by some flow rate measurements, by means of flow ratecontrollers D1, D2, D3, by at least one oxygen online analyzer close toD3, and by some control valves V1 and V2 for example, which controls theproportion of oxygen to be added to the air and the composition of theoxygen-enriched air. There is an alarm in case of problems in the oxygenconcentration, as the oxygen enrichment may vary between 20.95% and32.5% by volume, and more preferably between 25% vol and 30.5% vol. Suchalarm is connected to at least one safety valve V1, V2, V3, which isclosed if the proportion of oxygen is not comprised into this range, andwhich stop the process. Such online controllers, alarm, safety valves,are all connected to a control unit 50, which manages the regulation ofproportion of oxygen in air. For that, control unit determines all theset values of the gases ratios in the mixture and receives themeasurements from the online analyzer(s), compares the measurements withthe set values and manages all the safety devices if it detects aninconsistency in the results of the comparisons.

A static mixer 16 is preferably used to mix the obtained oxygen-enrichedair in order to homogenize its composition.

Installation 100 comprises a second unit, referenced U2 on FIG. 3, foradding simultaneously methane-containing gas and ammonia into theoxygen-enriched air. Methane and ammonia have to be simultaneously addedto the oxygen-enriched air for avoiding explosion. For that, the volumicratio of methane to ammonia in the reactant gas mixture has to beprecisely controlled in the range between 1.02 to 1.35, depending on theproportion of oxygen into oxygen-enriched air. Thus, if the proportionof O₂ into O₂-enriched air is 20.95% by volume, then the ratio ofCH₄/NH₃ is 1.35 and if the proportion of O₂ into O₂-enriched air is32.5% by volume, then the ratio of CH₄/NH₃ is 1.02.

As in the first unit U1, there is at least three flow rate measurements,by means of flow rate controllers D5, D6, D7, and some control valves V5and V6 connected to the control unit 50, which manages the regulation ofthe flow rates of each gas, and at least one safety valve V5, V6, V7 incase if the proportions of CH₄ and NH₃ are not in the optimal safetyoperational area.

Preferably, the second unit U2 is a premixing unit for pre-mixingmethane-containing gas with ammonia and for adding the obtainedpre-mixture into the oxygen-enriched air. Before adding the pre-mixtureof methane and ammonia into the oxygen-enriched airstream, thecomposition of said pre-mixture may also be homogenized in anotherstatic mixer 24. The addition of a homogeneous pre-mixture into thepre-mixture of oxygen-enriched air also advantageously avoids a localdetonation point. However, in order to preserve costs, such a staticmixer 24 may be not used.

A static mixer 30 is advantageously disposed on the feeding pipe,referenced P7, of the Andrussow type reactor 60, in order to homogenizethe composition of the resultant reactant gas mixture, before its entryinto the reactor 60. Such a homogenized composition, of the mixture ofresultant reactant gas, avoids a local detonation point.

The installation comprises preferably a static mixer (30) designed tohomogenize the composition of the reactant gases mixture before itsintroduction into the Andrussow type reactor (60) and advantageously itfurther comprises a second static mixer (16) designed to homogenize thecomposition of oxygen-enriched air before the addition of methanecontaining gas and ammonia.

For efficiency of the reaction the volumic ratio of oxygen-enriched airto ammonia is in the range between 7 (namely lower oxygen enrichment) to3.5 (namely larger oxygen enrichment).

For flammability reasons, methane and ammonia feed volumic ratios arelinked to the oxygen enrichment. More preferably, optimum parameters arelinked according to following equations:

Y1=1.4745X−0.1611, where Y1 is the molar NH₃/O₂-enriched air ratio and Xis the molar O₂/O₂-enriched air ratio, and

Y2=1.0406X−0.0234, where Y2 is the molar CH₄/O₂-enriched air ratio and Xis the molar O₂/O₂-enriched air ratio.

The control unit 50 is connected to an alarm and controls the ratios of(O₂+air)/(CH₄) and of (O₂+air)/(Ammonia) to avoid entering into aflammable zone. This range of proportion of oxygen is an economiccompromise to limit quantity of nitrogen from air to be heated up duringreaction and in the same time to limit pure oxygen or enriched airconsumption. This leads also to a reduction of the amount of energyrequired to heat. Preparing an enriched air brings also advantages toreduce methane consumption, to increase productivity with reasonablesize of equipment in reactor but also downstream the reactor, namelyammonia absorption column and HCN absorption column placed after theAndrussow type reactor.

Moreover, for safety reasons it is better first to heat each feed streamseparately. Thus, each gas stream is independently preheated before itspremixing with one another. Indeed, ammonia and methane-containing gasare independently preheated at a temperature between 60° C. and 100° C.,by respective heaters 23 and 21, and oxygen and air are independentlypreheated at a temperature between 100° C. and 165° C., by means ofrespective heaters 15 and 13. Then, air and oxygen are premixed tocontrol accurately enrichment. Ammonia and methane are premixed beforemixing with enriched air. Such preheating of each gas enables to obtaina—resultant reactant gas mixture, at the output of the static mixer 30,having a temperature advantageously comprised between 80° C. and 120°C., preferably between 95° C. and 115° C. Such preheating and premixingof the gases avoids detonable mixture between either enriched air andammonia or enriched air and methane. The temperature between 80° C. and120° C., preferably between 95° C. and 115° C. for the resultantreactant gas mixture is an optimal zone of temperature to be sure tohave a non-explosive gas mixture.

Moreover, the range of temperature between 80° C. and 120° C.,preferably between 95° C. and 115° C. of the resultant reactant gasmixture, at the output of the static mixer 30, is a good compromise toavoid large excess of methane, so to improve HCN/CH₄ yield, and also toavoid making a detonable mixture.

Methane-containing gas is usually a natural gas, which is extracted fromthe underground, but methane containing gas can also come from othersources like for example petroleum industry (refineries, steamcracking), from biogas industry (fermentation), from coal industry (coalmine, coking). For safety reasons and in particular, for avoidingdetonation and for avoiding damage of catalyst gauzes of the Andrussowtype reactor, the purity of the methane has to be above 94% by volume.Indeed, the natural gas extracted from the underground may comprisesulphur components. However, sulphur components are pollutants forcatalyst gauzes and reduce the number of active catalyst sites. It ishowever possible to clean the gas for purifying it and removing anyresidual concentration of sulphur components like mercaptans or hydrogensulphide to proportions less than 5 ppm for each component and less than20 ppm for total sulphur content. For that, a well-known desulfurizationprocess can be used.

Furthermore, it is preferable to follow the methane-containing gasquality because an excess of hydrogen leads to an increase of theflammability risk, because hydrogen has not the same properties asmethane in terms of burning and it is therefore difficult to control theflammability area. Moreover, an excess of alcene is pollutant for thecatalyst gauzes of the Andrussow type reactor 60. Typically, theproportion of hydrogen has to be less than 3%, and the concentration ofalkenes has to be less than 3500 ppm by volume. Consequently, an onlineanalyzer (not shown) is advantageously placed on the feeding pipe P3, inorder to follow the gas quality, especially the hydrogen and ethyleneconcentration.

According to another aspect of the invention, the resultant mixture ofreactant gases flowing in the feeding pipe P7 of the Andrussow typereactor 60 shall have a controlled flow rate. Indeed, the flow rate ofthis mixture has to be higher than a minimum threshold value in order toavoid a backfire from the catalyst gauzes of the reactor, whosetemperature is more than 750° C. Thus, the flow rate of the reactantgases mixture at the inlet of the Andrussow type reactor has to behigher than the flame velocity. The flame velocity mainly depends on thegas mixture composition, the temperature of the gas mixture and thefeeding pipe P7 diameter. All gases are mixed in a single pipe P7 andflow across the static mixer 30, which homogenizes the composition ofthe mixture before feeding the reactor 60.

According to another aspect of the invention, to avoid carbamatesformation between CO₂ (Carbone dioxide) from air and ammonia beforegauzes of the reactor's catalyst, temperature of gas mixture must beover 70° C. Otherwise, air shall be washed with cool water at atemperature comprised between 15° C. and 30° C. Such washing thusimproves catalyst gauzes life of the Andrussow type reactor.

Advantageously, steam may be added into the airstream before itspremixing with oxygen. Such steam addition helps avoiding coke formationon catalyst gauzes of the Andrussow type reactor 60 and improves theirlifetime. The steam addition enables also to moisten the air and toimprove reaction performance. Steam flow rate is also controlled by aflow rate controller (not shown).

Such washing or steam addition is schematized by the same blocreferenced 12 on the FIG. 3. It is advantageously placed upstream theair heater 13.

Finally, for efficiency of reaction and more particularly for avoidingpollution of catalyst gauzes with dust or metallic oxides, it is betterto provide a filtration device 10, 14, 20, 22 respectively on eachfeeding pipe P1 to P4. The resulting reactant gases mixture is alsofiltered by a filtration device 31. Such filtration devices on each gasstream allows to avoid pollution from pipes, of downstream processdevices, like the reactor 60 and its catalyst gauzes, by removing solidparticles, like iron for example, which is a precursor of coke. Indeed,iron oxide is a promoter for cracking methane, ammonia and also HCN.Thus, the decomposition reaction may lead to the formation of carbon(also called “coke”). After the last filtration 31, the gas mixture issent to the Andrussow type reactor 60 for the synthesis of hydrogencyanide HCN, at a temperature comprised between 750° C. and 1250° C.,preferably between 1000° C. and 1200° C. and at an absolute pressurebetween 1.4 and 3 bar, and preferably between 2.2 and 2.4 bar.

Preferably, all the devices and units of the installation 100, such aspipes, safety valves, online controllers, filtering devices, heaters,air compressor etc. . . . are made of stainless steel. Such material isbetter than steel in terms of protection of catalyst gauzes. Indeed,when steel is used, an iron oxide based passivation layer, which is apollutant for the catalyst gauzes, is formed in the devices. Suchpassivation layer does not appear with stainless steel.

The operating conditions that have been described above are a goodcompromise for improving HCN/CH₄ and HCN/NH₃ yields, while limitingoxygen amount relative to prior art and the related costs.

Synthesis of Monomer Selected from Methacrylic Acid and/or its Esters

FIG. 4 shows a simplified schematic block diagram of a plant for theproduction of methacylic acid and/or its esters from acetone andhydrogen cyanide HCN thus prepared by the Andrussow process. TheAndrussow process is for example described in the document U.S. Pat. No.1,934,838.

Such monomers can be further used to produce for examplepolymethylmethacrylate (PMMA) that is a polymer widely used in a lot ofapplications like for example automotive, transport, aerospace,photovoltaic, informatics, telecommunications, wind energy, or buildingconstruction . . . .

Preparation of Hydrogen Cyanide

First of all (steps S1 and S2) HCN is produced, from a mixture ofmethane-containing gas, ammonia and oxygen-enriched air. A mixture ofreactant gases is prepared (step S1), by means of the installationdescribed above, and introduced (step S2) into Andrussow type reactor 60comprising catalyst gauzes based on platinum/rhodium gauzes. Mixture ofgases passes over the catalyst gauzes and reacts at a temperaturecomprised between 750° C. and 1250° C., preferably between 1000° C. and1200° C. to form HCN. The oxygen-enriched air enables to increase theproductivity and to reduce the methane consumption. It facilitates alsoNH₃ absorption, HCN absorption and HCN distillation in downstreamcolumns. The HCN produced is quickly cooled and treated so as to avoidpolymerization of HCN. For that, ammonia which has not reacted isabsorbed by reaction with sulfuric acid, and the HCN is absorbed andstabilized in an absorption column, and then distilled in a distillationcolumn to reach a purity of 99.5% wt.

Preparation of Acetone Cyanohydrin

The thus synthesized HCN is then mixed with acetone (C₃H₆O), in stepreferenced S3, to produce acetone cyanohydrin ACH. The crude acetonecyanohydrin obtained is then purified by distillation.

Amidification of Acetone Cyanohydrin

In further steps (referenced S4, S5 in FIG. 4), the acetone cyanohydrinACH prepared in the precedent step S3 is subjected to an amidificationfor producing 2-methacrylamide (also called “MACRYDE”).

This amidification requires two steps S4 and S5 to reach the finalproduct. First, in step S4, sulphuric acid (H₂SO₄) is added in excess incomparison with acetone cyanohydrin ACH. The molar ratio of H₂SO₄/ACH iscomprised between 1.25 and 1.8, more preferably between 1.3 and 1.6.

The first reaction occurring is the hydrolysis of ACH by sulphuric acidwhich gives an intermediate salt called the SIBAM (forα-sulfatoisobutyramide). This reaction is the following:

(CH₃)₂COHCN+H₂SO₄→(CH₃)₂COSO₃HCONH₂ Acetone cyanohydrin (ACH)α-sulfatoisobutyramide (SIBAM)  (3)

This reaction is fast and exothermic. The temperature is around 80°C.-110° C. and the pressure is close to the atmospheric pressure.

The second reaction (step S5) is a slow and endothermic reaction. Itoccurs at atmospheric pressure and a temperature range between 110° C.and 165° C., preferably between 125° C. and 150° C. and more preferablybetween 130 and 145° C. This reaction is a cooking reaction which lastsbetween 3 and 16 minutes. This reaction is the following:

(CH₃)₂COSO₃HCONH₂→CH₂C(CH₃)CONH₂+H₂SO₄ α-sulfatoisobutyramide (SIBAM)methacrylamide (MACRYDE)  (4)

During the synthesis reactions there are many other by-products. Themain side reaction is described below.

The hydrolysis of ACH by water is very fast. A small quantity of watercan create a significant quantity of HIBAM (α-hydroxyisobutyramide).This reaction is the following:

(CH₃)₂COHCN+H₂O→(CH₃)₂COHCONH₂ Acetone cyanohydrin (ACH)α-hydroxyisobutyramide (HIBAM)  (5)

In step S5, HIBAM can also create MACRYDE, but this reaction is veryslow. So there is a large quantity of unconverted HIBAM at the end ofamidification step S5. The reaction is the following:

(CH₃)₂COHCONH₂→CH₂C(CH₃)CONH₂+H₂O α-hydroxyisobutyramide (HIBAM)methacrylamide (MACRYDE)  (6)

The hydrolysis of HIBAM may create HIBA (α-hydroxyisobutyricacid):

(CH₃)₂COHCONH₂+H₂SO₄+H₂O→(CH₃)₂COHCOOH+NH₄HSO₄ α-hydroxyisobutyramide(HIBAM) α-hydroxyisobutyricacid (HIBA)+ammoniumbisulfate  (7)

At the same time, a significant amount of methacrylic acid MAA isproduced by the hydrolysis of SIBAM by water. This reaction is thefollowing:

(CH₃)₂COSO₃HCONH₂+H₂O→CH₂C(CH₃)COOH+NH₄HSO₄ α-sulfatoisobutyramide(SIBAM) methacrylic acid (MAA)+ammoniumbisulfate  (8)

The mixture of MACRYDE and MAA obtained after amidification is theneither hydrolyzed (step S7), by adding water to the MACRYDE, oresterified (step S6), by adding methanol to the MACRYDE.

Esterification

Then the components obtained after amidification, namely themethacrylamide and the methacrylic acid, are esterified, so as to obtainmethylmethacrylate MMA. The esterification reaction (step S6) is made bymixing said components with methanol (CH₃OH).

The main reactions are these two below:

CH₂C(CH₃)CONH₂+H₂SO₄+CH₃OH→CH₂C(CH₃)COOCH₃+NH₄HSO₄ methacrylamide(MACRYDE)+methanol methylmethacrylate (MMA)+ammoniumbisulfate  (9)

CH₂C(CH₃)COOH+CH₃OH→CH₂C(CH₃)COOCH₃+H₂O methacrylic acid (MAA)+methanolmethylmethacrylate (MMA)  (10)

Hydrolysis

The components obtained after amidification can also be hydrolysed bymixing them with water (step S7). Such hydrolysis reaction allows toobtain methacrylic acid according to the following reaction:

CH₂C(CH₃)CONH₂+H₂SO₄+H₂O→CH₂C(CH₃)COOH+NH₄HSO₄ methacrylamide (MACRYDE)methacrylic acid (MAA)+ammoniumbisulfate  (11)

Purification of Crude MMA or MAA Obtained

Either the crude methylmethacrylate MMA obtained after esterification(S6) or the crude methacrylic acid MAA obtained after hydrolyse (S7) isthen purified (step S8) by classical process known in the art, in orderto remove residual compounds.

1. An installation (100) for the preparation of hydrogen cyanide HCN,said installation comprising: A) an Andrussow type reactor (60), inwhich is introduced a reactant gases mixture of methane-containing gas,ammonia and oxygen-enriched air, said mixture reacting inside saidreactor (60) over a catalyst to produce HCN, B) upstream of saidAndrussow type reactor (60) according to the direction of gas flow, gasfeeding pipes (P1 to P4) for feeding the installation with each reactantgas, a first unit (U1) for pre-mixing oxygen with air before addingmethane-containing gas and ammonia into the obtained pre-mixture ofoxygen-enriched air, wherein said first unit (U1) comprises at least oneoxygen online controller so as to control the ratio of oxygen into air,said ratio of oxygen to air comprising between 20.95% and 32.5% involume of oxygen; C) a second unit (U2) for adding simultaneouslymethane-containing gas and ammonia into the oxygen-enriched air, with avolumic ratio of methane to ammonia, being between 1.35 and 1.02depending on the proportion of oxygen into the oxygen-enriched air; andD) at least one heat exchanger for heating at least one of the reactantgases, in order to have a temperature of said reactant gases mixturefeeding said Andrussow type reactor (60) comprised between 80° C. and120° C.
 2. Installation according to claim 1, wherein said the secondunit (U2) is a premixing unit for premixing the methane-containing gaswith ammonia and adding the obtained pre-mixture into theoxygen-enriched air.
 3. Installation according to claim 1, wherein saidinstallation further comprises a static mixer (30) designed tohomogenize the composition of the reactant gases mixture before itsintroduction into the Andrussow type reactor (60).
 4. Installationaccording to claim 3, wherein said installation further comprises asecond static mixer (16) designed to homogenize the composition ofoxygen-enriched air before the addition of methane containing gas andammonia.
 5. Installation according to anyone of claim 4, wherein saidinstallation further comprises a third static mixer (24) designed tohomogenize the pre-mixture of methane containing gas and ammonia beforeits introduction into oxygen enriched air stream.
 6. Installationaccording to anyone of claim 1, wherein said installation furthercomprises one heat exchanger (13, 15, 21, 23) placed respectively oneach gas feeding pipe (P1 to P4) of the installation (100) for heatingeach gas stream separately, before its pre-mixing with another gas. 7.Installation according to claim 1, wherein said installation furthercomprises one filtration device (10, 14, 20, 22) placed respectively oneach gas feeding pipe (P1 to P4) of the installation (100), and onefiltration device (31) placed on the feeding pipe (P7) of the Andrussowtype reactor (60).
 8. Installation according to claim 1, wherein saidinstallation further comprises a washing device (12) designed to washthe air before its premixing with oxygen when the temperature of saidair is lower than 70° C., said washing being made with water, thetemperature of which being comprised between 15° C. and 30° C. 9.Installation according to claim 1, wherein said installation furthercomprises a steam injection device (12) designed to inject steam intothe air before its pre-mixing with oxygen.
 10. Installation according toclaim 1, wherein said installation further comprises a control unit(50), able to manage a regulation of the flow rate of each reactant gasaround a predetermined set value, said control unit being connected toat least one flow rate controller, placed on each gas stream, and to atleast one safety valve, that is closed in case of a discrepancy betweenset value and measurements made by said flow rate controller. 11.Installation according to claim 1, wherein all the devices, units andpipes of the installation (100) are made of stainless steel.
 12. Aprocess for preparing hydrogen cyanide HCN by reacting, over a catalystplaced inside an Andrussow type reactor (60), a reactant gases mixturecontaining a methane-containing gas, ammonia and oxygen-enriched air,the process comprising before feeding the Andrussow type reactor withthe reactant gases mixture, a pre-mixing step of oxygen with airfollowed by a simultaneous addition of methane-containing gas andammonia into the obtained pre-mixture of oxygen-enriched air, saidprocess comprising the steps of pre-mixing oxygen with air with a ratiocomprised between 20.95% and 32.5% by volume; simultaneously addingmethane containing gas and ammonia in the pre-mixture of oxygen-enrichedair in such a manner that the volumic ratio of methane to ammonia iscomprised between 1.35 and 1.02 depending on the content of oxygen intoair; said obtained reactant gases mixture having a temperature comprisedbetween 80° C. and 120° C. for feeding the Andrussow type reactor (60).13. The process according to claim 12, wherein the content of oxygen inair is progressively increased until reaching the maximum value of 32.5%by volume.
 14. The process according to claim 12, wherein the volumicratio of oxygen-enriched air to ammonia is in the range between 7 to3.5.
 15. The process according to claim 12, wherein the respectivevolumic ratios of ammonia and methane to oxygen-enriched air are linkedaccording to the following equations:Y1=1.4745X−0.1611, where Y1 is the molar ratio of NH₃/(O₂-enriched air)and X is the molar ratio of O₂/(O₂-enriched air), andY2=1.0406X−0.0234, where Y2 is the molar ratio of CH₄/O₂-enriched air)and X is the molar ratio of O₂/(O₂-enriched air).
 16. The processaccording to claim 12, wherein before pre-mixing, each gas isindependently preheated, ammonia and methane-containing gas beingindependently preheated at a temperature between 60° C. and 100° C.;oxygen and air being independently preheated to a temperature between100° C. and 165° C., so that the resultant reactant gas mixture, feedingthe Andrussow type reactor, is at said temperature between 80° C. and120° C.
 17. The process according to claim 12, wherein the flow rate ofthe reactant gases mixture is controlled before feeding the Andrussowtype reactor (60) so that it is higher than a predetermined thresholdvalue, which depends on a flame velocity.
 18. The process according toclaim 12, wherein the methane-containing gas comprises methane of purityabove 94% by volume.
 19. The process according to claim 12, whereinbefore the preheating step and the premixing step of air with oxygen,and if the temperature of air is less than 70° C., air is washed withwater, whose temperature is comprised between 15° C. and 30° C.
 20. Theprocess according to claim 12, wherein before the preheating step andthe premixing step of air with oxygen, steam is injected into airstream.
 21. The process according to claim 12, wherein each gas streamand the mixture of resultant reactant gases is filtered before feedingof Andrussow type reactor (60).
 22. A process for preparing a monomerselected from methacrylic acid and/or methylmethacrylate comprising thesteps: preparing (S1-S2) hydrogen cyanide HCN by an Andrussow process,preparing (S3) acetone cyanohydrin ACH from hydrogen cyanide andacetone, hydrolyzing (S4) acetone cyanohydrin ACH to produce anhydrolysis mixture comprising α-hydroxyisobutyramide (HIBAM),α-sulfatoisobutyramide (SIBAM), 2-methacrylamide (MACRYDE) andmethacrylique acid (MAA), thermally converting (S5) said hydrolysismixture into a cracker mixture to produce a mixture of 2-methacrylamide(MACRYDE) and methacrylic acid (MAA), reacting the cracker mixture witha material selected from methanol (S6) or water (S7) to producerespectively a monomer selected from methyl methacrylate (MMA) ormethacrylic acid (MAA), and purifying (S8) said obtained methylmethacrylate (MMA) or methacrylic acid (MAA), wherein the first step(S1) of the preparation of hydrogen cyanide HCN is processed by usinginstallation (100) according to claim 1.